The present invention relates to processes for reducing the concentration of selenium and/or arsenic in wastewater or drinking water. More particularly, the present invention is directed to water purification processes using an iron-loaded cation exchange resin to separate selenium and arsenic contaminants in the form of cation exchange resin immobilized iron(III) selenite and iron(III) arsenate complexes.
Selenium is a Group 6A nonmetal found in nature in the -2, 0, +4, and +6 oxidation states. Selenite (Se(IV)) and selenate (Se(VI)) are the most common forms occurring in groundwater. Although selenium is believed to be an essential trace element, it is highly toxic to humans and livestock in large doses. Accordingly the EPA has established a low maximum contaminant level for selenium in drinking water (10 micrograms/liter as of 1989), creating the need for the development of effective selenium removal technologies.
In addition to natural selenium contamination of groundwater and surface water, potential selenium pollution from industrial sources is significant. Selenium finds use in the manufacture of a wide variety of industrial products including pigments, glass, rubber, insecticides, and electronic components. Selenium is frequently present in wastewater streams from petroleum refineries, coal-fired power plants, and mining operations. Clean up of wastewater streams from such sources presents substantial technical challenges.
Numerous water treatment methods have been employed to achieve acceptable removal of selenium from wastewaters. Lime softening, conventional coagulation, activated carbon, and activated alumina adsorption methods have all been attempted, with mixed results.
Ion exchange has also been studied. Removal of selenium using a strong-base anion exchange resin, for example, is described in U.S. Pat. No. 4,915,928 to Marcantonio. Tanaka et al., Selective Collection of Selenium (IV) From Environmental Water By Functionalized Ion Exchange Resin, describes the use of an anion exchange resin in connection with bismuthiol-II and azothiopyrinsulfonic acid (ATPS) as terfunctional reagents. Boegel et al., EPA Report No. 600/2-86/031 (1986), recommends a two-step process of oxidation of selenium-containing water with free chlorine followed by strong-base anion exchange. Some anion exchange techniques have been unsuccessful due to their inability to handle sulfur-laden wastewater streams. Selenium and sulfur have very similar chemical properties, including similar affinity for anion exchange resins. Thus, sulfur may occupy a majority of the resin binding sites to the exclusion of selenium, rendering the anion exchange resin ineffective.
Arsenic is a 5A nonmetal found in nature in the -3, 0, +3, and +5 oxidation states. Arsenite (III) and arsenate (V) are the most common forms found in drinking water and wastewater streams. Thus, as in the case of selenium contaminants, health and environmental concerns regarding arsenic contaminants in wastewater and drinking water streams have stimulated significant research and development of efforts to define water purification techniques for arsenic removal. There exists a need for new water purification technologies that can provide cost effective commercially practicable removal of selenium and arsenic from contaminated water streams.
In accordance with one embodiment of the present embodiment there is provided a process for treating a wastewater or drinking water stream contaminated with selenium to reduce the concentration of selenium contaminants. When the selenium contaminant is primarily in the form of selenite, the process comprises contacting the stream with a cation exchange resin, preferably a strong acid cation exchange resin, loaded (complexed) with iron(III). The selenite anions react with the resin complexed iron cations to form iron(III) selenite complexes immobilized on the resin surface. To optimize selenium removal as selenite, it is preferred that the process include the step of treating the water or wastewater stream with a reducing agent to convert a major portion of the non-selenite forms of selenium contaminants to selenite before contact with the iron(III)-complexed cation exchange resin. Thus when a selenite stream is, for example, passed through a bed of an iron(III)-complexed strong cation exchange resin, iron-selenite forms as an immobilized ionic complex on the surface of the cation exchange resin resulting in a treated effluent stream having a reduced selenium concentration. The iron complexed cation exchange resin can be readily regenerated for use in accordance with the invention by contacting it firstly with an acid (to release the iron selenite complex and form the acid form of the resin) and thereafter with a solution of a water soluble iron salt.
In accordance with another aspect of the present invention, a method is provided for reducing arsenic concentration in a wastewater or drinking water by contacting the stream with an iron(III)-complexed cation exchange resin, preferably a strong acid cation exchange resin, to form an iron(III) arsenate complex immobilized on the cation exchange resin and an effluent stream having reduced arsenic concentration. Preferably the arsenic contaminated water or wastewater stream is treated with an oxidant to convert a major portion of the arsenite forms of arsenic to arsenate prior to contacting the stream with the iron(III)-complexed cation exchange resin.
The present invention provides an efficient and easily managed process for removing selenium and arsenic components from water or wastewater streams by taking advantage of the formation and immobilization of ionic complexes of iron(III) selenite and arsenate on the complexed-iron bearing surfaces of the cation exchange resin. Thus in accordance with the present invention ionic complexes are formed by reaction of the respective contaminant anions with the exchange resin complexed cationic iron, and are thereby immobilized on the surface of the cation exchange resin. The iron-complexed cation exchange resin can be easily regenerated in a resin bed or resin column configuration by consecutive treatment with acid and a solution of a soluble iron salt.
There is also provided in accordance with this invention a water treatment apparatus for reducing the concentration of selenium and arsenic in contaminated water streams. The apparatus comprises a strong acid cationic exchange resin complexed with iron(III), means for contacting the contaminated water stream with the ion exchange resin, and, optionally, means for pretreating (reducing or oxidizing respectively) the contaminated water stream before contact with the iron-complexed resin.
Additional objects, features and advantages of the invention will become apparent to those skilled in the art upon consideration of the following detailed description of the preferred embodiments exemplifying the best mode of carrying out the invention as presently perceived.